Conventionally, a substrate glass for various displays, particularly, those for depositing a metal or oxide thin film and the like on the surface has been required to have the following characteristics.
(1) Having a very low content of alkali metal oxide, specifically, having a content of alkali metal oxide of 1,000 mass ppm or less, because, if alkali metal oxide is contained, alkali metal ions diffuse in a thin film to deteriorate film characteristics.
(2) Having small deformation of a glass substrate due to heating in a thin-film deposition process, particularly, having small thermal shrinkage; that is, having a small thermal shrinkage rate.
(3) Having sufficient chemical durability against various chemicals used in semiconductor formation; in particular, having durability against buffered hydrofluoric acid (BHF: mixed liquid of hydrofluoric acid and ammonium fluoride) for etching SiOx or SiNx, against a chemical solution containing hydrochloric acid used for etching of ITO, against various acids (nitric acid, sulfuric acid, etc.) used for etching of an metal electrode, and against an alkaline of a resist removing liquid.(4) Having no defects (bubbles, striae, inclusions, pits, flaws, etc.) in the inside and on the surface.
In addition to the above requirements, the recent situations are as follows.
(5) Reduction in weight of a display is required, and the glass itself is also required to be a glass having a small density.
(6) Reduction in weight of a display is required, and a decrease in thickness of the substrate glass is desired.
(7) In addition to conventional amorphous silicon (a-Si) type liquid crystal displays, polycrystal silicon (p-Si) type liquid crystal displays requiring a slightly high heat treatment temperature have come to be produced (a-Si: about 350° C.; p-Si: 350° C. to 550° C.).(8) In order to improve productivity and increase thermal shock resistance by increasing the rate of rising and falling temperature in heat treatment for manufacturing a liquid crystal display, a glass having a small average thermal expansion coefficient is required.
Meanwhile, drying in etching has been progressed, and a requirement for BHF resistance has been reduced. As the conventional glass, in order to improve BHF resistance, a glass containing from 6 to 10 mol % of B2O3 has been largely used. However, B2O3 has a tendency of reducing strain point. Example of a non-alkali glass which does not contain B2O3 or has a small content include the followings.
Patent Document 1 discloses a glass which contains from 0 to 3 weight % of B2O3. However, the strain point in Examples is 690° C. or lower.
Patent Document 2 discloses a glass which contains from 0 to 5 mol % of B2O3. However, the average thermal expansion coefficient at from 50° C. to 350° C. is more than 50×10−7/° C.
In order to solve the problem in the glasses disclosed in Patent Documents 1 and 2, a non-alkali glass disclosed in Patent Document 3 has been proposed. The non-alkali glass disclosed in Patent Document 3 has a high strain point, is capable of being formed by a float method, and is suitable for a use of a substrate for a display, a substrate for a photo mask and the like.